Tell her that you'll go with her to speak with the Bishop and get the answer straight from him. Does she not believe in forgiveness and the power of repentence? Yes, I know, usually, we are hardest on ourselves and all too often don't forgive ourselves when we should. But that's all part of the repentance process. Depending on the reasons she left, there may be some unfinished business that needs to be taken care of, or there may not be. but the only way she'll get the answer she is seeking, is to speak to someone holding the authority to answer her questions. If speaking with Bishop seems overwhelming for her at this point, perhaps start with the missionaries. That's one of the reasons they are there, remember.
If she is sincere in her repentance, and I have no reason to doubt her sincerity, then I am sure she will be welcomed with open arms, much as the prodigal son was. Perhaps you can re-read that story to her? :)
